A unified constitutive model for pressure sensitive shear flow transitions in moderate dense granular materials

Granular shear flows exhibit complex transitional regimes that are dramatically affected by the pressure level and shear stress state. New advances in granular shear tests at low pressure have enlightened the understanding of the two granular shear flow transitions: between quasi-static and moderate shear flows, and between steady-state and transient shear flows. However, a unified constitutive model to describe these two transitions is yet to develop. In this work, a simplified and unified model is proposed based on innovative triaxial shear flow tests, using two dimensionless physical variables. Model results validated against experimental data suggest that the shear flow transition between a quasi-static to a moderate Isotach type flow state is highly pressure-dependent. At extremely low pressure, the granular viscosity becomes the primary mechanism, suppressing the quasi-static mechanism even under “quasi-static” shear rates. In transient to steady state granular flow transitions, a mobilized shear stress ratio or mobilized friction coefficient between zero and the critical state ratio for consolidated granular packings is taken into consideration. This is coupled with the mechanism of granular viscosity. These findings have not been discussed before and are of great relevance to granular mechanics as well as space and earthquake engineering.

Micro-slips in an experimental granular shear band replicate the spatiotemporal characteristics of natural earthquakes

Memory effects in seismology—such as the occurrence of aftershock sequences—are implicitly assumed to be governed by the time since the main event. However, experiments are yet to identify if memory effects are structural or time-dependent mechanisms. Here, we use laser interferometry to examine the fluctuations of deformation which naturally emerge along an experimental shear fault within a compressed frictional granular medium. We find that deformation occurs as a succession of localized micro-slips distributed along the fault. The associated distributions of released seismic moments, as well as the memory effects in strain fluctuations and the time correlations between successive events, follow exactly the empirical laws of natural earthquakes. We use a methodology initially developed in seismology to reveal at the laboratory scale the underlying causal structure of this behavior and identify the triggering kernel. We propose that strain, not time, controls the memory effects in our fault analog.